1. Field of the Invention
The present invention relates to silicon carbide light emitting diodes having a p-n junction, and more particularly, to light emitting diodes which can emit blue light with high luminance.
2. Description of the Related Art
Since a light emitting diode is a small luminescent source which consumes a small amount of power and can stably emit light with high luminance, it is used as a variety of display apparatus and a light source for reading recorded information. Light emitting diodes capable of emitting visible light of a color in the range of red to green have been put into practical use. On the other hand, light emitting diodes capable of emitting blue light are now under development and have not yet attained light emission with sufficient luminance for practical use.
In general, the colors of light emitted from light emitting diodes depend on the semiconductor materials used. Semiconductor materials for blue light emitting diodes are limited to silicon carbide (SIC) that is a IV--IV group compound semiconductor, gallium nitride (GAN) that is a III-V group compound semiconductor, and zinc selenide (ZnSe) and zinc sulfide (ZnS) that are II-VI group compound semiconductors.
For the structure of light emitting diodes using the above-mentioned semiconductor materials, a p-n junction structure is most suited, where electrons and holes can be injected in a light emitting region with high efficiency. However, it is difficult to use any of GaN, ZnSe, and ZnS among the above semiconductor materials for the fabrication of light emitting diodes having a p-n junction. This is because it is difficult to obtain p-type crystals from these semiconductor materials, or, even if these crystals are obtained, they have high resistance or are very unstable.
To avoid this problem, light emitting diodes with a metal-insulator-semiconductor (MIS) structure comprising a metal layer and a thin insulating layer in addition to any of the above semiconductor materials have been fabricated. However, light emitting diodes with the MIS structure possess the disadvantages of having non-uniform device characteristics and providing unstable light emission.
On the other hand, silicon carbide can be used as a material for the p-n junction type light emitting diodes, because both p-type crystals and n-type crystals can easily be obtained. Silicon carbide includes a variety of polytypes of which band gaps are slightly different from one another. Typically, a 6H-type silicon carbide having a band gap of approximately 3 eV at room temperature is used for blue light emitting diodes.
One of the conventional methods for fabricating p-n junction type blue light emitting diodes using silicon carbide is liquid phase epitaxy (LPE). Many reports have already been made on light emitting diodes fabricated by LPE (see, for example, M. Ikeda, et al, Journal of Applied Physics, vol. 50, No. 12, 1979, pp. 8215-8225).
A blue light emitting diode using donor-acceptor pairs composed of nitrogen donors and aluminum acceptors as a luminescent center has recently been reported (Y. Matsushita, et al., Japan Society of Applied Physics, vol. 60, No. 2, 1991, pp. 159-162). As is shown in FIG. 13, the light emitting diode comprises an n-type epitaxial layer 9 doped with nitrogen as the donor and a p-type epitaxial layer 3 doped with aluminum as the acceptor, which are successively formed on an n-SiC substrate 1 in this order. The n-type epitaxial layer 9 is also doped with a small amount of aluminum, thus to form a donor-acceptor pair. This light emitting diode emits blue light having a peak wavelength of 470 to 480 nm.
However, the conventional light emitting diodes fabricated by LPE still emit light with low luminance. When they are driven at an operation current of 20 mA, for example, the luminance is only 15 mcd or less. The principal reason for this low luminance is considered to be as follows. The growth temperature of p-type and n-type crystals is as high as 1700.degree. C. to 1800.degree. C., so that the crystal growth takes place in active molten silicon, thereby making it difficult to accurately control the crystal growth, and presenting a great possibility that unnecessary impurities will enter the growing crystals. Moreover, there is the disadvantage that the use of LPE does not allow for the mass production of blue light emitting diodes.
A method for fabricating p-n junction type light emitting diodes by chemical vapor deposition (CVD) has been disclosed by the inventors, in which stable emission of blue light with high luminance can be obtained. According to this method, the crystal growth is controlled with high accuracy and mass production of the light emitting diodes is possible (Japanese Patent Application No. 2-129918). The inventors have also proposed another type of light emitting diode fabricated by CVD (Japanese Patent Application No. 2-184464 and No. 2-406598, and A. Suzuki, et al., Abstracts ICVGE-7 (The International Conference on Vapour Growth and Epitaxy, Nagoya, Japan), 1991, p. 111).
The proposed light emitting diode has a structure as is shown in FIG. 14, which comprises an n-SiC single-crystal layer 2 doped with nitrogen as the donor impurity and a p-SiC single-crystal layer 3 doped with aluminum as the acceptor impurity formed successively on an n-SiC substrate 1 in this order. This light emitting diode effects light emission by free exciton recombination or by acceptor-related recombination, which changes depending on the concentration of the donor impurities. The peak wavelength of this light emitting diode is 455 nm for the light emission caused by acceptor-related recombination and 425 nm for the light emission caused by free exciton recombination.
U.S. Pat. No. 4,918,497 discloses a light emitting diode in which donor-acceptor pairs composed of nitrogen donors and aluminum acceptors are present in a p-type layer. This light emitting diode has a structure as is shown in FIG. 15, which comprises an n-type layer 2, a p-type layer 4 which emits blue light by pairs of nitrogen donors and aluminum acceptors, and a p-type layer 5 doped with only aluminum acceptors, which are successively formed on an n-SiC substrate 1 in this order. The purpose of forming the p-type layer 5 is to send a current to a wider area of the p-n junction by using the low resistance of this layer, and light is not emitted from this layer. The light emitting diodes shown in FIGS. 13, 14, and 15 are all provided with a p-sided ohmic electrode 6 and an n-sided ohmic electrode 7.
In order to realize visible light emitting diodes with high luminance, the efficiency of human eyes to light, or visibility, must be considered. The relationship between the visibility and the color of emitted light is shown in FIG. 9. The visibility is highest when the light is green having an wavelength of around 555 nm. The visibility lowers as light shifts to the blue side with shorter wavelengths and to the red side with longer wavelengths. This means that a color of light with lower visibility is perceived as being darker by human eyes even though the light output is the same. Therefore, the visibility increases when the wavelength of blue light is closer to 555 nm. However, when the wavelength of blue light is closer to 555 nm, the blue color is hued with green and loses blue monochromaticity. Generally, the optimal peak wavelength of blue light with good visibility and excellent blue monochromaticity is 460 to 470 nm.
Neither of the aforementioned silicon carbide blue light emitting diodes have attained this peak wavelength. The light emitting diodes shown in FIGS. 13 and 15, both of which use pairs of nitrogen donors and aluminum acceptors, have a peak wavelength of 470 to 480 nm. Blue light with wavelengths in this range is advantageous in visibility, but it is mixed with green, thus lowering blue monochromaticity. The light emitting diode shown in FIG. 14 which includes a p-SiC single-crystal layer doped with aluminum acceptors has a peak wavelength of 455 nm. The blue light with this wavelength is excellent in blue monochromaticity, but it is disadvantageous in visibility.